A foil air bearing is a bearing which supports loads by air pressure generated by the incoming air having viscosity between a rotor or a bearing disc and a foil contacting therewith at high speed rotation of the rotor.
Among the foil air bearing, the journal foil air bearing may generate the dynamic instability of the rotor-bearing system at high speed. That is, when rotor rotates at high speed, the dynamic characteristic becomes unstable and problems such as vibration may arise.
The dynamic instability may be caused by the unintended pressure distribution between the rotor and the foil and be affected by the friction damping of the foil structure or the unbalanced mass of the rotor. This dynamic instability harms the stable operation of the rotor and should be solved.
Generally, the journal foil air bearing is composed of one bump foil and one top foil or three bump foils and three top foils.
Among them, it is known that the journal foil air bearing composed of the three bump foils and three top foils is advantageous in view of the load capability and dynamic stability. However, in this case, the number of bearing components increases and the production cost rises, the three bump foils and the three top foils should have all the same tolerance, and all the fixed portions of the three top foils are exposed to the rotor so that the reliability of the fixed portion should be secured.